In the manufacturing of thermal sensitive recording papers, a leuco chlomogen and a color developer, which are both colorless or pale-colored, are normally coated on supporting materials such as papers. In the process for the manufacturing of a coating solution for such use, a color developer is dispersed into an aquatic solution of water-soluble polymer being a binding agent to prepare the coating solution. Since thermal sensitive recording papers are required to have a property of high sensitivity to temperature, it is necessary to make the particle size of a color developer in the dispersible solution thereof fine as much as possible in order to produce thermal sensitive recording papers having excellent temperature-sensitive property. For this reason, most of dispersible solutions of color developers wherein a color developer is grinded up to fine particles, have been produced by adding the color developer into an aquatic solution of a water-soluble polymer and subsequently grinding them throughly by employing certain wet grinding method.
Although 4-hydroxy-4'-isopropoxydiphenylsulfone can function as an excellent color developer and has been widely used, the compound sometimes develops crystals thereof either during the process of wet grinding to prepare a dispersible solution of the color developer or during the preservation of the dispersible solution of the color developer prepared, if the color developer was grinded up to fine particles by employing the means as described above. Particularly, this phenomenon becomes more conspicuous when 4-hydroxy-4'-isopropoxydiphenylsulfone is grinded up to a particle size of less than 1 .mu.m. It is understood that this development of crystals is brought as a result of hydration phenomenon of 4-hydroxy-4'-isopropoxydiphenylsulfone and the crystals developed are the hydrate of 4-hydroxy-4'-isopropoxydiphenylsulfone. This hydrate seriously relates to a disadvantageous property of thermal sensitive recording papers such as unclean background, because the thermal sensitive recording papers tend to result in uncleanness of the background of the recording papers if they are produced in using a dispersible solution of a color developer which contains such hydrate.
As a means for preventing the generation of said crystal development (hydration), a method to add 4,4'-dihydroxydiphenylsulfone into 4-hydroxy-4'-isopropoxydiphenylsulfone to thereby prevent the generation of crystals development, is disclosed in Japanese Patent Laid-opened No. Hei 2-76779. By even employing this method for the production of thermal sensitive recording papers, however, the recording papers frequently give said unclean background when the adding rate of 4-4'-dihydroxydiphenylsulfone is raised to 5% more or less.
It is an object of the present invention to provide an improved means for preventing the generation of said hydration of 4-hydroxy-4'-isopropoxydiphenyls ulfone in the process of the production of a dispersible solution of color developer and during the storing thereof.